Crosslinked nylon block copolymers

ABSTRACT

A crosslinked nylon block copolymer comprising a copolymer containing a polyamide block and an elastomeric block, irradiation crosslinked, including a compound which promotes crosslinking therein. In process form, the present invention comprises supplying a nylon block copolymer, containing a polyamide block and an elastomeric block, along with crosslinking promotor and exposing the block copolymer to irradiation, sufficient to crosslink the copolymer and improve mechanical properties therein. The crosslinked nylon block copolymers here have particular utility in both the medical and wire and cable industries.

FIELD OF THE INVENTION

The present invention relates to the preparation of crosslinked nylonblock copolymer materials, and to their use in medical applications,particular as a component for the preparation of a rigidized stemsection for a catheter or other similar medical device. In addition, thecrosslinked nylon block copolymer materials herein have utility in thewire and cable industry.

BACKGROUND OF THE INVENTION

Generally, nylon block copolymers may be alternating blocks of polyamidesegments and other segments such as segments of elastomeric polymerssuch as polyethers, polyesters, hydrocarbons or polysiloxanes. Thesenylon block copolymers are generally prepared by copolymerizing a lactammonomer in the present of the elastomeric polymers component. A moredetailed discussion of the structure and method of preparing particulartypes of nylon block copolymers can be found in U.S. Pat. No. 4,031,164.

The polyamide segments and elastomeric polymer segments of the nylonblock copolymers each contribute to the respective properties of thefinal polymer. In order to obtain high modulus materials, polyamidesegments of higher molecular weight and/or higher weight percent can beemployed. Alternatively, greater tensile elongation and impactproperties, as well as lower surface hardness, may be obtained by usinghigher percents of and/or higher molecular weight elastomeric polymercomponent.

U.S. Pat. No. 4,671,355 appears to be one of the first disclosures of acrosslinked nylon block copolymer, chemically, through the use ofpolyfunctional amine compounds. That is, the crosslinked nylon blockcopolymers are prepared by a reaction scheme in which polyfunctionalamines act as crosslinking agents. More specifically, a crosslinkedmaterial was reportedly synethesized by reacting an acyl lactamfucntionalized material with the polyfunctional amine to preparecrosslinked acyl lactam materials which were then concurrently orsubsequently reacted with lactam monomer in the presence of a lactampolymerization catalyst to form the crosslinked nylon block copolymermaterial. It was reported therein that by chemically crosslinking, itwas discovered that the overall properties of the final polymer could bevaried even if one maintains the molecular weight and weight percent ofthe elastomeric polymer component.

While the above chemical method of crosslinking a nylon block copolymerhas been reported, no reports exist concerning the development of anylon block copolymer by a more convenient method such as irradiation.The closest attempts in this regard can be found, for example, in Plast.Massy, 1993, No. 2, pp 35-37, which contains a paper entitled"Production and Properties of Crosslinked Compositions of AliphaticNylons". According to the abstract, a study was conducted on the processof radiation crosslinking of an aliphatic polyamides (i.e., not a nylonblock copolymers) and an assessment is made of the properties andnetwork compositions obtained. The materials studied were nylon-6,nylon-6,6 and nylon-12. The polyfunctional monomers employed toaccelerate crosslinking were triallyl cyanurate and triallylisocyanurate. Mechanical data is supplied.

Similarly, in the Chinese Journal of Polymer Science, Vol. 7, No. 1,there is a paper entitled "Characterization of Irradiated CrystallinePolymer-Isothermal Crystallization Kinetics of Radiation InducedCrosslinked Polyamide 1010". As disclosed therein, after irradiation,the service temperature of the resin is raised to about 240° C. Inaddition, network formation is said to greatly change thecrystallization behavior of the otherwise crystalline polyamidematerial.

Finally, it is worth noting that various other disclosures have beenuncovered, which recite thermoset (or crosslinked) polyamide resins, butagain, no mention or suggestion of irradiation crosslinking of a nylonblock copolymer is described. For example, in U.S. Pat. No. 5,198,551entitled "Polyamide Thermosets" there is disclosed what is termedcurable polyamide monomers, curable liquid crystal polyamide monomersand thermoset compositions prepared therefrom. The thermoset polyamidesso prepared all contained highly aromatic type structure. Similarly, inU.S. Pat. No. 5,3154,011, which is a divisional of the '551 Patent,there is again described curable polyamide monomer systems, whichmonomers represent highly aromatic type functionality.

The fact that there have been no reports concerning the development of aconvenient route for the preparation of a crosslinked nylon blockcopolymer is underscored when reference is made to U.S. Pat. No.5,584,821, which discloses an angiographic catheter which has arelatively stiff though flexible shaft and a soft tip. The soft tipconsists primarily of a tungsten loaded polyether block amide (PEBA)copolymer surrounded by two thin PEBA layers. This three ply radiopaquetip is bonded to a PEBA shaft. The shaft is reinforced either by aninner nylon ply or by metal braiding.

In other words, pursuant to the teachings of U.S. Pat. No. 5,584,821when it comes to the production of a soft tip catheter with a relativelystiffer body, the teachings therein emphasize that the stiffer bodyportion relies upon the use of a metal braided reinforced PEBA copolymeror a co-extruded two ply wall consisting of nylon and PEBA copolymer.That being the case, it becomes clear that inasmuch as PEBA typecopolymers are widely used in catheter type applications, it would servea long-standing need if one could conveniently produce a more rigid andtoughened PEBA catheter, without the need for the structuralmodifications emphasized in the prior art.

Accordingly, it is an object of this invention to prepare a crosslinkednylon block copolymer, wherein said polymer is conveniently crosslinkedby the process of irradiation or other high energy source, wherein suchcrosslinked nylon block copolymer has particular utility as a componentof a medical catheter product.

More specifically, it is object of the present invention to prepare acrosslinked nylon block copolymer elastomeric formulation, viairradiation techniques, wherein the elastomeric composition, subsequentto crosslinking, exhibits improvement in properties such as mechanicalstrength, heat resistance, and hardness, and in particular, thecrosslinked material so produced demonstrates elongational behavior whenexposed to elevated temperatures under conditions of constant stress.

Furthermore, it is an object of the present invention to crosslink nylonblock copolymer systems, wherein such crosslinking improves the overallelastomeric toughness of the block copolymer, thereby providing what canbe termed a much more durable nylon block copolymer product for avariety of miscellaneous applications in the medical industry.

SUMMARY OF THE INVENTION

A crosslinked nylon block copolymer comprising a copolymer containing apolyamide block and an elastomeric block, irradiation crosslinked,including a compound which promotes crosslinking therein. In processform, the present invention comprises supplying a nylon block copolymer,containing a polyamide block and an elastomeric block, along withcrosslinking promotor and exposing said block copolymer to irradiation,sufficient to crosslink said block copolymer and improve mechanicalproperties therein, particularly the ability of said block copolymer toelongate upon exposure to a constant load of about 29 psi at an elevatedtemperature of about 200° C. for 15 min.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the present invention in composition form relates to aan irradiation-crosslinked nylon block copolymer. Preferably, the nylonblock copolymer is a nylon block copolymer sold by ATOCHEM under thetradename PEBAX® which is a elastomeric type nylon block copolymer. Thecommercial PEBAX polymers consist of polyether blocks separated bypolyamide blocks. The polyether blocks may be based upon polyethyleneglycol, polypropylene glycol, or polytetramethylene ether glycol. Thepolyamides are usually based upon nylon-11 but may be based upon nylons6 of nylon-6,6 or even a copolymer such as nylon-6/nylon-11. A widerange of block polyamides have been offered and vary in the type ofpolyether, the nature of the polyamide block and the ratio of polyetherto polyamide blocks. The polymers range in hardness from Shore A 60 toShore D72 which is broader than for the thermoplastic polyester andthermoplastic polyurethane rubbers. Melting range is also dependent onthe particular composition, and varies between 140-215° C.

The above nylon block copolymers have been found to undergo crosslinkingupon exposure to irradiation. Listed below in Table I are the results ofvarious exposure levels as applied to a PEBAX Shore 72A material,containing 2.0% TAIC (triallylisocyonurate) and the correspondingchanges in mechanical properties observed:

                  TABLE I                                                         ______________________________________                                                0 MR  5 MR     10 MR   15 MR  20 MR                                   ______________________________________                                        T.S. (psi)                                                                              8,632   10,531   7,751                                                Yield (psi) 4,026 4,979 5,186                                                 100% Mod (psi) 3,498 4,035 4,216                                              Elong (%) *404.2 *358.3 *283.3 ** **                                          Creep (%) *** 54.3 61.4 58.3 63.0                                             Set (%) -- 1.5 4.1 3.6 4.6                                                  ______________________________________                                         *= Necked                                                                     **=NOTE: The 15 mr and 20 mr samples necked and there was very little         elongation (less than 1/4 inch).                                              ***At 200° C., the unexposed material melted. At 150° C. th     unexposed material elongated 1/32 inch (3.1%). AT 175° C. it broke     in the clamp, but did not melt. The sample elongated 5/8 inch (62.5%)         before it broke.                                                         

One of the more relevant properties reported on in Table I, is the %Creep which was measured at 200° C., 29 psi, over a 15 minute period.This is formally known as the "Test Method for Measurement of Hot Creepof Polymeric Insulations", Publication T-28-562, published by theInsulated Cable Engineers Association, Inc, of South Yarmouth, Mass. Inaccordance with the present invention, elongations of less than 100% arepreferred, and most preferred is an elongation of about 10-65%.

As can be seen, unexposed PEBAX actually melted under these conditionsof testing, and no elongation was observed. By contrast, after a 5megarad total exposure, the percent creep is about 54.6%. In otherwords, irradiation clearly promotes crosslinking and network formationwithin the nylon block copolymer material, and as a thermoset, it nolonger melts and flow, and elastomeric behavior is observed.

Also, as can be seen from Table I, exposure to 5 megarads results in anassociated drop in the percent of elongation from about 404% to about358%, which is a characteristic expected due to crosslinking. Inaddition, exposure at 5 megarads increases the tensile strength fromabout 8600 psi to about 10,500 psi, which is again a result of networkcrosslink formation.

As can also be seen in Table I, while an exposure of 5 megarads alongwith about 2.0% of a promotor ("TAIC" or "TAC", triallylcyanurate)provides optimum composition and conditions, higher exposure levels arestill acceptable. For example, a total exposure of 10 megarads similarlyprovides a sample that elongates about 61% after exposure to 29 psi, at200° C. for 15 minutes. However, with respect to this particular sample,it is worth pointing out that the tensile strength drops to about 7750psi, which may be the onset of some degradation. Upon exposure to evenhigher total levels of irradiation (15 and 20 megarads) the sample stilldemonstrates elongational values of about 58 and 63%, respectively,however, at such total exposure levels, the samples necked and their wasvery little elongation at room temperature. Again, this is believed tobe the result of the degradation that may take place when total becomeshigh.

With regards to the specific utility of the invention disclosed herein,it is noted that the crosslinked nylon block copolymer disclosed hereinhas utility in both the medical products field, as well as in the wireand cable industry.

More specifically, when it comes to the production of an intravascularflexible catheter having a tubular shaft comprising a nylon blockcopolymer, and a soft flexible tubular tip distal of and bonded to saidshaft, the improvement recited herein comprises irradiation crosslinkingsaid nylon block copolymer of said shaft, wherein said crosslinkingincreases the rigidity of said shaft relative to said soft flexibledistal tip. In addition, in the balloon catheter field, in the case ofsuch catheters manufactured from a nylon block copolymer, the inventionherein provides for the preparation of a balloon type catheter, whereinthe balloon section relative to the shaft can be converted into athermoset or crosslinked type structure, thereby increasing its overallmechanical strength, performance, and durability.

Accordingly, the compositions and method disclosed herein provide a muchmore convenient route for the preparation of a novel rigid-flex nylonblock copolymer resin, particularly suited for the production of noveltype catheter products, without the need for structural modification ofthe catheter type systems as disclosed and emphasized by the prior art.In addition, the compositions herein are well-suited as an electricalinsulating material for the wire and cable industry.

We claim:
 1. A crosslinked nylon block copolymer comprising a copolymercontaining a polyamide block and an elastomeric block, irradiationcrosslinked, including a compound which promotes crosslinking therein,wherein said compound which promotes crosslinking comprisestriallylisocyanurate or triallylcyanurate.
 2. A crosslinked nylon blockcopolymer of claim 1 wherein the triallylisocyanurate ortriallylcyanurate is present at level of about 2.0% by weight.
 3. Thecrosslinked nylon block copolymer of claim 1, wherein said crosslinkednylon block copolymer elongates after about 15 minutes at about 200° C.and about 29 psi.
 4. The crosslinked nylon block copolymer of claim 3,wherein said elongation is less than about 100%.
 5. The crosslinkednylon block copolymer of claim 3, wherein said elongation is about10-65%.
 6. The crosslinked nylon block copolymer of claim 1, whereinsaid elastomeric block is selected from a polyether, polyester,hydrocarbon, polysiloxane, or mixtures thereof.
 7. The crosslinked nyloncopolymer of claim 1, wherein said polyamide block is a nylon-6,nylon-6,6, nylon-11, copolymer of nylon-6/nylon-11, or mixtures thereof.8. The nylon block copolymer of claim 1, wherein said irradiation isless than about 20 megarads.
 9. The nylon block copolymer of claim 1,wherein said irradiation is 5, 10, 15, or 20 megarads.